This invention relates generally to apparatus for aligning a semiconductive wafer with a photomask and exposing a photosensitive film on a surface of the wafer through the photomask. It relates, more particularly, to an improved chuck for holding the wafer parallel to, but out of contact with, the photomask while the wafer is being aligned with and, in some apparatus, while it is being exposed through the mask.
Many prior art devices are available for aligning a semiconductive wafer with a photomask, and a number of such devices are described in U.S. Pat. No. 3,858,978 issued Jan. 7, 1975 to Karl-Heinz Johannsmeier. As explained more fully in that patent, it is desirable to maintain the photomask and semiconductive wafer parallel, but separated, during the alignment and exposure steps. For that reason, out-of-contact or proximity chucks such as the one described in the aforementioned patent have been developed.
A proximity chuck must perform several functions in addition to holding the semiconductive wafer to its top surface. The chuck must be depressible below a reference surface, and some mechanism must be provided to hold the chuck in position without slippage. In addition, the holding mechanism must not impart any movement to the chuck while the chuck is being held in position. It is also desirable for the chuck to be able to move about axes in the horizontal plane. This angular movement compensates for wafers that are slightly warped or wedge-shaped. However, in addition to holding or maintaining the vertical position of the chuck, the holding mechanism must also be able to hold the angular position of the chuck.
As is more fully described in the referenced prior patent, a commonly used prior art holding mechanism was a plastic or metal spring that frictionally held a chuck plate within a base member. It has been discovered, however, that this type of holding mechanism has a number of disadvantages. After a period of use the spring and the mating surface on the base member will wear, reducing the amount of frictional holding force and allowing undesirable slippage. In these prior art devices, very close tolerances in dimensions and material quality must be maintained to produce a consistent holding force. In addition, wear patterns such as grooves or slight depressions formed on the mating surface can cause the spring to hold the chuck plate at the same position each time it is depressed, even when a slightly different position is required. Furthermore, the prior art proximity chucks did not hold the chuck plate from angular movement about axes perpendicular to the direction of displacement of the chuck.
Another type of holding mechanism that has been proposed uses spring clutches attached to a chuck plate. Each spring clutch surrounds a post on a base member, and an actuating mechanism is attached to the free end of the spring. To hold the position of the chuck plate, the actuating mechanism pulls on the free end of the spring to tighten it around the post. This type of holding mechanism, however, has the significant disadvantage that it usually imparts some undesirable movement to the chuck plate during the tightening of the spring.